Cheapest Bubble Tea in Montreal?
If price is such a hindrance, you could always make it yourself!!! Just get the balls at P.A. or any asian grocery store and mix with your favourite juice (I recommend Cool Quenchers). I'm happy to pay ~5$ at L2 for some quality tapioca. When I did find some cheap ones below 4$, I was usually very disappointed.
You can get a very authentic Taiwanese bubble tea made without powder that uses white gourd tea and syrup at Su Shian Yuang on Rachel. With tapioca balls it is around $4 but much better than the usual. Hakata Ramen has a bubble tea with exploding tapioca balls that I like but some tapioca purists protest. Same kind of price.
Taro is a good flavour to try if you are having a powdered one and it is worth nothing that the milk powder is non-dairy. The cheapest might be on the Main or in Chinatown? If you are really cheap you can buy the tapioca balls and powder at Sakaris on St-Laurent or Marche Hawai, but it is hard to make well.